Paging method and apparatus

ABSTRACT

A two-way paging system utilizes four local frequencies for transmissions between pager units ( 22 ) and a central control station ( 20 ). A first local frequency (f 1 ) carries a local clock; a second local frequency (f 2 ) carries communications packets from the central control station to paging units; a third local frequency (f 3 ) carries communication packets from the pager units to the central control station; and a fourth local frequency (f 4 ) carries a status or request signal from the paging units ( 22 ) to the central control station ( 20 ). Transmissions on the fourth local frequency (f 4 ) are in accordance with a time divided slot allocation among pager units accessing the central control station ( 20 ). For a two-way paging system having a plurality of central control stations ( 420   x ) servicing a corresponding plurality of cells, a total of eight frequencies are utilized within any one cell. Four of the utilized frequencies are the local frequencies (f 1 -f 4 ) [which may differ from cell to cell], and four of the utilized frequencies are lower power common frequencies or switching frequencies (C 1 -C 4 ) which are used to switch or hand-off a pager unit ( 422 ) traveling from one cell to another.

BACKGROUND

[0001] 1. Field of Invention

[0002] This invention pertains to communications paging, andparticularly to two-way paging method and apparatus.

[0003] 2. Related Art and Other Considerations

[0004] Over the last several decades, pagers have proven to be importantcommunication devices for contacting remotely situated personnel.Whereas primitive pagers provided primarily only a tonal and/orvibratory output, more modern pagers have enhanced output capabilitiessuch as message-bearing alphanumeric displays.

[0005] Paging systems have historically been one-way systems. That is,the user receives a paging message from a central terminal but has noway of responding to that message with the pager. Prior art attempts toprovide two-way communication capabilities for a pager have includedefforts to connect the pager to a telephone (e.g., to a mobile radiotelephone). See, for example, U.S. Pat. No. RE 33,417 to Bhagat et al.(which combines an entire radio pager and radiotelephone linked throughan automatic dialer) and U.S. Pat. No. 5,117,449 to Metroka, et. al.(which purports to combine paging and cellular radiotelephone functionsin a single unit).

[0006] Some pagers have the capability of providing an acknowledgment orresponse to a paging signal. In some such “ack-back” systems, a useroperates a reply input device (e.g., a toggle switch, pushbutton switch,or keyboard) when paged. Typically such ack-back systems involve acomplex acknowledgement transmission scheme, involving numerousfrequencies or frequency sub-bands. Hand-off of the pager, as the pagertravels between differing geographic regions or “cells” served bydiffering central stations, becomes technically cumbersome whenmultitudinous frequencies are involved.

SUMMARY

[0007] A two-way paging system utilizes four local frequencies fortransmissions between pager units and a central control station. A firstlocal frequency carries a local clock; a second local frequency carriescommunications packets from the central control station to paging units;a third local frequency carries communication packets from the pagerunits to the central control station; and a fourth local frequencycarries a status or request signal from the paging units to the centralcontrol station. Transmissions on the fourth local frequency are inaccordance with a time divided slot allocation among pager unitsaccessing the central control station.

[0008] For a two-way paging system having a plurality of central controlstations servicing a corresponding plurality of cells, a total of eightfrequencies are utilized within any one cell. Four of the utilizedfrequencies are the local frequencies, (which may differ from cell tocell), and four of the utilized frequencies are lower power commonfrequencies or switching frequencies which are used to switch orhand-off a pager unit traveling from one cell to another.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments as illustrated in the accompanyingdrawings in which reference characters refer to the same partsthroughout the various views. The drawings are not necessarily to scale,emphasis instead being placed upon illustrating the principles of theinvention.

[0010]FIG. 1 is a schematic view of a central control station includedin a paging system of an embodiment of the invention.

[0011]FIG. 2 is a schematic view of a pager unit included in a pagingsystem for use with the central control station of FIG. 1.

[0012]FIG. 3 is a flowchart depicting steps executed by the centralcontrol station of FIG. 1.

[0013]FIG. 4 is a flowchart depicting steps executed by the pager unitof FIG. 2 when in a transmit mode.

[0014]FIG. 5 is a flowchart depicting steps executed by the pager unitof FIG. 2 when in a receive mode.

[0015]FIG. 6 is a timing diagram reflecting communications between thecentral control station of FIG. 1 and the pager unit of FIG. 2.

[0016]FIG. 7 is a schematic view of a central control station includedin a paging system of a second embodiment of the invention.

[0017]FIG. 8 is a schematic view of a pager unit included in a pagingsystem for use with the central control station of FIG. 7.

[0018]FIG. 9 is a hybrid schematic view and timing diagram forrepresenting switching operations for the paging system of the secondembodiment of the invention.

[0019]FIG. 10 is a flowchart depicting steps executed by the pager unitof FIG. 8 in connection with a channel switching operation.

[0020]FIG. 11 is a flowchart depicting steps executed by the centralcontrol station of FIG. 7 in connection with a channel switchingoperation.

[0021]FIG. 12 is a schematic view of a format of a communications packetutilized with embodiments of the invention.

[0022]FIG. 13 is a schematic view illustrating a time divided slotallocation technique according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 shows a central control station 20 according to a firstembodiment of the invention; FIG. 2 shows a paging unit 22 suitable foruse with central control station 20.

[0024] As shown in FIG. 1, central control station 20 includes centralcomputer 30; transmitter 32; receiver 34; and computerized telephoneanswering system 36. Transmitter 32 transmits, via transmitting antenna42, two local frequencies, namely frequency f₁ and frequency f₂.Receiver 34 is connected to receiver antenna 44 for reception of twolocal frequencies, namely frequency f₃ and frequency f₄. Computerizedtelephone answering system 36 is connected to a bank of telephones 48.

[0025] Central computer 30 of central control station 20 comprises aconventional computer equipped with typical components including a CPU50; I/O interface 52; and memory 54. Although shown only generally inFIG. 1, it should be understood that memory 54 includes a number ofunillustrated memory devices, including (for example) a hard disk drive,RAM, and ROM. FIG. 1 shows that memory 54 has stored therein (amongother things) a pager registration file 55 and a pager directory file56. Pager files 55 and 56 are typically stored on a hard disk drive ofcentral computer 30, and upon start-up are loadable into a RAM portionof memory 54.

[0026] Central computer 30 of central control station 20 furtherincludes a decoder 57 (connected between receiver 34 and I/O interface52 for decoding in-coming communications information from one or morepager units 22), as well as encoder 58 (connected between I/O interface52 and transmitter 32 for encoding out-going communicationsinformation).

[0027] Central control station 20 also includes a clock unit 59 whichgenerates a local clock signal f₁clk (which, in turn, is used tomodulate frequency f₁).

[0028] As illustrated further herein, CPU 50 of central control station20 prepares communications packets for transmission on frequency f₂. Asgenerally illustrated in FIG. 12, the communications packets are of apredetermined format, having fields for identification of the centralcontrol station, for identification of the addressed pager unit(s) 22,for an operation code, for (optionally) alphanumeric information, andfor other conventional packet-type information such as checksum, errorcorrection, and postamble. The preamble and postamble are speciallychosen patterns which can be recognized and distinguished from data forthe purpose of determining the beginning and ending of a packet. Thealphanumeric information can be in a customary binary 8-bit format. Theformat of FIG. 12 is illustrative only, as such information as the orderof the fields can be varied in other embodiments.

[0029] Central control station 20 communicates with a plurality of pagerunits 22 ₁, 22 ₂, . . . 22 _(N). Only one such pager unit, genericallyreferenced as pager unit 22, is specifically illustrated and describedherein, it being understood that the construction and operation of otherpager units may be similar to the one illustration.

[0030] As shown in FIG. 2, pager unit 22 includes a pager receiverantenna 60 which is connected to pager receiver 62. Pager receiver 62is, in turn, connected through S/D converter 64 within pager computer70. Receiver 62 receives the two local frequencies f₁, and f₂, whichfrequencies have been modulated to carry in-coming communicationsinformation (described in more detail below) to pager computer 70. On acommunications output side, pager computer 70 outputs out-goingcommunications information to pager transmitter 72 via D/S converter 74.Transmitter 72 broadcasts, on pager antenna 76, the out-goingcommunications information on the two local frequencies f₃ and f₄.

[0031] As also shown in FIG. 2, pager computer 70 includes pagermicroprocessor 80 which is connected to each of an arithmetic processor;a memory system 84 (including both ROM and RAM); and I/O interface 86.I/O interface 86 is connected to a clock unit 87. I/O interface 86 isalso connected to receive in-coming decoded communications informationfrom an 8-bit decoder 88 and to output out-going uncoded communicationsinformation to an 8-bit encoder 90. Decoder 88 is connected to receivein-coming coded communications information from S/D converter 64;encoder 90 is connected to output out-going coded communicationsinformation to D/S converter 74.

[0032] Clock unit 87 is settable by suitable inputs thereto so thatclock unit 87 generates a local clock signal f₁clk having a frequencycorresponding to its input. It should be understood that, in otherembodiments, the function of clock unit 87 can be performed at leastpartially by microprocessor 80 using programmed execution.

[0033] I/O interface 86 is also connected to supply an on/off signal online 92 to pager transmitter 72, as well as to facilitate input andoutput with numerous input/output devices. The input/output devicesconnected to I/O interface 86 include keyboard 93; beeper 94; vibrator95; and LCD (alphanumeric) display 96.

[0034] Upon manufacture, pager unit 22 is preprogrammed with anidentification serial number (e.g., a 7-digit alphanumeric pre-assignedID number) which is stored in memory 84 (ROM). Pager unit 22 isactivated (e.g., at the time of purchase) by inserting a time slotassignment (explained below) both into a predetermined address in memory84 of pager unit 22 and into pager directory file 56 (stored in memory54 of central control station 20).

Operation of First Embodiment

[0035] Communication between central control station 20 and pager unit22 occurs on the four local frequencies, in particular the frequenciesf₁, f₂, f₃, and f₄ mentioned above. The first frequency (f₁) carries thelocal clock-aligning signal from central control station 20 to pagingunit 22. The second frequency (f₂) carries a pager command andalphanumeric data from central control station 20 to paging unit 22. Thethird frequency (f₃) carries pager status data and alphanumeric datafrom. paging unit 22 to central control station 20. The fourth frequency(f₄) carries a pager request signal from paging unit 22 to centralcontrol station 20. In the illustrated embodiment, the frequencies f₁-f₄are preferably chosen so that f₁≠f₂≠f₃≠f₄.

[0036] As explained in more detail below and illustrated in FIG. 13, innormal non-cell-switching operation, the pager request signal onfrequency f₄ is transmitted in a predetermined time slot assigned topaging unit 22. The predetermined time slot on frequency f₄ is relatedto the. clock-aligning signal (carried by frequency f₁) and assignedwhereby the fourth frequency is utilizable by a plurality of otherpaging units. For example, as shown in FIG. 13, a first time slot onfrequency f₄ is assigned to a pager P1; a second time slot is assignedto page P2, and so on up to time slot n assigned to pager Pn. In theillustrated embodiment, the number of time slots (and accordingly thenumber of pagers) may be as many as ten thousand or more.

[0037]FIG. 3 shows steps executed by CPU 50 of central control station20 in processing communications to and from one or more paging units.The steps depicted in FIG. 3 are indicative of instructions stored in aROM portion of memory 54 of central control station 20.

[0038] When central control station 20 is started up (step 100), aninitialization process (step 102) is conducted. Included in theinitialization process is activation of transmitter 32 (so thattransmitter 32 can transmit at the two frequencies f₁ and f₂) andactivation of receiver 34 (so that receiver 34 can receive the twofrequencies f₃ and f₄). Moreover, frequency f₁ is modulated to carry thelocal clock-aligning signal generated by local clock 59. Then, at step104, the pager registration file 55 and the pager directory file 56 areloaded from hard disk into a RAM section of memory 54 (step 104).

[0039] After initialization and loading of the files 55 and 56, CPU 50repetitively executes an instruction loop 106. Loop 106 involveschecking to determine (at step 108) whether a telephone message is beingreceived (via answering system 36 from one of the telephones in bank 48)and checking to determine (at step 110) whether a pager message is beingreceived (via transmitter 32 from one of the pager units 22).

[0040] As used herein, a message, whether originated from a telephone orfrom a pager, may require a plurality of packets for transmission from acentral station 20 to a pager 22 or vise versa. In the ensuingdiscussion, transmission and reception of messages subsumes transmissionand reception one or more packets. In general, the packetization ofmessages will be invisible to the user, meaning that a user enters amessage without regard to the number of packets which might be requiredto transmit the message. The message typically ends with a user-enteredmessage termination character or message delimiter character. Thetransmitting device (either central station 20 or pager 22), allocatesthe message to one or more packets having a format similar to that ofFIG. 12, with the last packet in the message bearing the messagetermination character. Alternatively, the packets may be formatted in amanner to indicate the number of consecutively related packets emanatingfrom a transmitter (e.g., there may be a separate packet fieldindicating the continuation number of related packets).

[0041] Central computer 30 can distinguish between receipt of atelephone message (at step 108) and a pager message (at step 110) byvirtue of the fact that I/O interface 52 generates different type ofinterrupts to CPU 50 depending on the type of message received. If it isdetermined at step 108 that a telephone message is being received, steps112, 114, and 116 of FIG. 3 are executed.

[0042] In processing a received telephone message, at step 112 centralcomputer 30 extracts out-going communications information from thepredeterminately sequenced telephone-entered data. The telephone-entereddata, entered via a touchpad of a calling one of the telephones in bank48, includes by convention an identification (e.g., telephone number) ofthe calling telephone; an identification of the called pager unit (e.g.,the 7-digit alphanumeric pre-assigned ID number); and any character datafor transmission followed by a termination character. This out-goingcommunications information is received at central computer 30 instandard DTMF format.

[0043] At step 114, using the ID number of the called pager (obtained atstep 112) central computer 30 checks the pager registration file 55 anddirectory file 56 to determine whether the called pager unit isregistered with central control station 20. Assuming that the calledpager is so registered, at step 114 the central computer 30 also obtainsfrom pager directory file 56 the slot assignment for the called pagerunit.

[0044] At step 116, central control station 30 transmits communicationsinformation to the called pager unit. In this regard, central controlstation 20 prepares and transmits (on frequency f₂) a communicationsmessage which includes, among other things, the ID of the called pagerunit and the character data received from the telephone for transmissionof the pager unit 22. After step 116 is executed, processing returns toloop 106.

[0045] If it is determined at step 110 that a pager message is beingreceived, even numbered steps 132-140 of FIG. 3 are executed (prior toreturning to loop 106). As will be seen hereinafter with respect to FIG.4, a sending pager unit 22 transmits, in its assigned time slot, arequest signal on frequency f₄ when the sending pager unit 22 desires tosend a message. As central control station 20 is always monitoringfrequency f₄, a request signal carried by frequency f₄ from any pagerunit 22 is noted. With reference to the local clock 59, at step 132 CPU50 determines in what time slot on frequency f₄ the request signal isdetected. Upon detection of the time slot at step 132, at step 134 CPU50 consults the pager directory file 56 to determine the identificationnumber of the particular pager unit 22 which originated the requestsignal.

[0046] With the identity of the requesting pager unit 22 now known, atstep 136 central control station 20 authorizes the requesting pager unit22 to transmit its message. In particular, CPU 50 directs preparation ofa communications message for transmission on frequency f₂. Theparticular communications packet prepared at step 136 includes anidentification of the requesting pager unit (the addressee of thepacket), as well as an operation code (“op” code) whichcommands/authorizes the requesting pager unit 22 to send its message.

[0047] At step 138, central control station 20 receives a communicationsmessage on frequency f₃ sent from the sending (e.g., requesting) pagerunit 22. The communications message prepared and sent by the sendingpager unit 22 includes packets of similar format to that shown in FIG.12, and includes an identification of a pager to which the message isultimately addressed as well as its own identification. At step 138, CPU50 checks to ensure that the ultimate addressee pager unit is registeredin pager files 55 and 56. At step 140, CPU 50 makes any necessaryreformatting and/or information substitution in the message, and causesthe message to be transmitted on frequency f₂. The transmission onfrequency f₂ required by step 140 includes the identification of theultimate addressee (e.g., a pager unit 22) as well as an operation codeindicating that the transmission includes a relayed message from anotherpager unit.

[0048] Steps executed by a pager unit 22 in connection with itstransmission mode are depicted in FIG. 4. Steps executed by a pager unit22 in connection with its receive mode are depicted in FIG. 5. The term“mode” as used herein does not connote exclusivity at any particularmoment, for it should be remembered that at all times pager unit 22 isreceiving transmissions on frequencies f₁ and f₂.

[0049] In its transmission mode (see FIG. 4), after start-up (step 200)microprocessor 80 of the transmitting pager unit 22 executes a loop 202wherein user alphanumeric characters (entered via keyboard 93) arerepetitively fetched (at step 204) until an end of message delimiter isdetected (at step 206). As entered, the characters fetched at step 204are displayed on LCD display 96. Entry of the delimiter character atstep 206 causes microprocessor 80 to exit loop 202. By convention, themessage must include an addressee ID, which addressee ID is likely theID of another one of the pager units to which the message entered instep 204 is directed.

[0050] After entry of the message awaits entry from keyboard 93 of atransmit command at step 212. Assuming that the transmit command isentered at step 212, microprocessor 80 prepares and sends a requestsignal on frequency f₄. As indicated before, the request signal istransmitted on frequency f₄ in a time slot assigned to the requestingpager unit 22. It should be kept in mind that pager unit 22 is all thewhile receiving the local clock-aligning signal on frequency f₁, whichenables microprocessor 80 to cause transmission of the request signal onfrequency f₄ at a time corresponding to the specific time slot allottedto the particular sending pager unit 22.

[0051] In the above regard, in accordance with time division techniques,each pager unit 22 ₁-22 _(N) (e.g., pagers P₁-P_(n) in FIG. 13) isassigned a selected one of N number of time slots on frequency f₄.

[0052] After transmission of the request signal at step 214, pager unit22 awaits receipt of a transmit command from central control station 20.Preparation and transmission of the transmit command/authorization fromcentral control station 20 is described with reference to FIG. 3. Uponreceipt of the transmit command/authorization from central controlstation 20 (step 216), microprocessor 80 prepares (at step 218) acommunications message with one or more packets having a format muchlike that of FIG. 12. The addressee ID and alphanumeric field of packetsof the communications message is filled with the message entered in loop202. At step 220, the sending pager unit 22 broadcasts thecommunications packet on frequency f₃.

[0053] If a transmit command is not entered at step 212, or aftertransmission of the message at step 220, microprocessor 80 awaits entryof at least one of several possible special function keys at step 222.For example, the user may press a function key which requires storage ofthe message (whether yet transmitted or not) [see step 228].Alternatively, the user may press function keys which facilitate editingor erasure of the message (see steps 224 and 226, respectively). Tocomplete the message and begin work on another message, a specialfunction key for an exit operation (step 230) must be pressed.

[0054]FIG. 5 depicts steps executed by microprocessor 80 of pager unit22 when in a receive mode. After start-up (step 302), and as indicatedby step 304, pager unit 22 receives transmissions from central controlstation 20 on frequency f₂. Once a complete packet is received(determined at step 306), a check is made (at step 308) whether theaddressee ID in the communications packet (see packet format of FIG. 12)is the ID of the receiving pager unit 22. If the determinations ofeither step 306 or 308 are negative, pager unit 22 awaits eithercompletion of the communications packet (in the case of step 306) orreceipt of another communications packet (in the case of step 308) bylooping back to step 304.

[0055] Assuming that the received communications packet is designatedfor this particular receiving pager unit 22, at step 310 microprocessor80 consults the operation code field of the communications packet (seeFIG. 12) to determine if the operation code indicates that the messageincludes a command. If the operation code indicates a command, a commandprocessing routine (framed by broken lines 312 in FIG. 5) is executed.

[0056] Assuming for the moment that the operation code does not indicatea command, at step 314 microprocessor 80 of pager unit 22 stores thealphanumeric field portion of the communications packet (which at leastpartially forms the message) in a RAM portion of memory 84. Since amessage communicated from central processing station 20 may requireseveral communications packets for completion of the message (withsubsequent communication packets providing continuations of the messagecontent), microprocessor 80 checks at step 316 to ensure that the entiremessage has been received. If not, processing continues back at step 304for reception of a further communications packet.

[0057] Upon reception of an entire communications message, at step 318microprocessor 80 determines whether pager unit 22 is in a beep mode ora vibrate mode. In this regard, there are numerous ways of settingpaging unit 22 to the desired mode, either by a specially dedicatedswitch on paging unit 22 or by data entry using keyboard 93. If pagerunit 22 is in a beep mode, microprocessor 80 outputs a signal whichcauses I/O interface 86 to issue a further signal to activate beeper 94(step 320). Alternatively, if pager unit 22 is in a vibrate mode,microprocessor 80 outputs a signal which causes I/O interface 86 toissue a further signal to activate vibrator 95 (step 322).

[0058] At step 324, microprocessor 80 directs I/O interface 86 to sendthe alphanumeric message data to LCD display 96, so that the receivedmessage can be viewed by the user.

[0059] After notification to the user (either via beeper 94 and/orvibrator 95), and display (on LCD 96) of the received alphanumeric data,microprocessor 80 returns to step 304 to check whether furthercommunications packets are being received.

[0060] The command processing routine (framed by broken lines 312 inFIG. 5) first determines (step 330) which particular operation is beingcommanded. This determination is based on the content of the operationcode, which is different for different command types. If the operationcode indicates an error shut-down, execution jumps to an error shut-downsub-routine which begins at step 340. If the operation code indicates atime slot change, execution jumps to a change time slot sub-routinewhich begins at step 350. If the operation code requires transmittershut-down, execution jumps to a transmitter shut-down sub-routine whichbegins at step 360. If the operation code requires transmitterre-enablement, execution jumps to a transmitter reenable sub-routinewhich begins at step 370. If the operation code requires clock re-set,execution jumps to a clock re-set sub routine which begins at step 380.

[0061] In connection with the error shut down sub-routine, at step 342microprocessor 80 obtains an indication of error type from thecommunications packet. The error type is stored in memory 84 (step 344)and then displayed on LCD display 96 (step 346). Then microprocessor 80issues a command (at step 348) to shut down pager unit 22, whichshut-down occurs at step 349.

[0062] In connection with the time slot changing sub-routine, at step352 microprocessor 80 extracts, from the received communications packet,information indicative of the new time slot assigned to the receivingpager unit 22. The new time slot is entered (at step 354) into memory 84and thereafter utilized (until further change) in connection withtransmission of request signals on frequency f₄ (see, for example, step214 of FIG. 4).

[0063] In connection with the transmitter shut down sub-routine, at step362 microprocessor 80 directs I/O interface 86 to issue an OFF commandto transmitter 72. In connection with the transmitter re-enablesub-routine, at step 372 microprocessor 80 directs I/O interface 86 toissue an ON command to transmitter 72.

[0064] In connection with the clock re-set sub-routine, at step 382microprocessor 80 directs that clock 59 of pager unit 22 be set.

[0065] After execution of steps 354, 362, 372, or 382, executioncontinues back to step 304 for processing of potential furthercommunications packets. Thus, unless an error shut-down is noted, eachentry of the command processing routine (framed by broken lines 312 inFIG. 5) is followed by a loop back to step 304.

[0066]FIG. 6 is a timing diagram showing the frequencies f₁-f₄ andintegration of the steps depicted in FIGS. 3-5, particularly in thecontext of a request by a sending pager unit P1 for sending a message toa sendee pager unit P2. As employed in FIG. 6, “computer” refers tocentral control station 20. It should be understood that the sendingpager unit P1 and the sendee pager unit P2 operate in both thetransmission mode as depicted in FIG. 4 and in the receiver mode asdepicted in FIG. 5. In general, FIG. 6 shows transmission of a messagefrom pager unit P1 (via central control station 20) to pager unit P2;transmission of a confirmation message from pager unit P2 (via centralcontrol station 20) to pager unit P1; and transmission of a message frompager unit P1 to central control station 20 indicating that pager unitP1 received the confirmation message from pager unit P2.

Structure of Second Embodiment

[0067]FIG. 7 shows a central control station 420 according to a secondembodiment of the invention; FIG. 8 shows a paging unit 422 suitable foruse with central control station 420.

[0068]FIG. 9 shows a wide area paging system including a plurality ofcentral control stations S1-S8 (each identical to central controlstation 420), each preferably geographically centered within arespective cell. Each central control station S1-S8 broadcasts its ownlocal frequencies, as well as a set of common or switching frequenciesC₁-C₄. The common frequencies C₁-C₄ are broadcast at a lower power, sothat reception thereof occurs only in a relatively small neighborhood orcommon frequency reception region (CFRR) [also referred to as a“switching region”] about the central control station. The localfrequencies are broadcast at a significantly greater power for receptionsubstantially throughout the cell. For example, in FIG. 9, centralcontrol station S1 broadcasts its lower power common frequencies C₁-C₄to CFRR₁ and its higher power local frequencies f₁-f₄ to CELL; centralcontrol station S2 broadcasts its lower power common frequencies C₁-C₄to CFRR₂ and its higher power local frequencies f₅-f₈ to CELL₂.

[0069] As also shown in FIG. 9, CELL₁ and CELL₂ overlap in an overlapregion shown in FIG. 9. Station S1 utilizes a set of local frequenciesf₁-f₄; station S2 utilizes a different set of local frequencies f₅-f₈.Both stations S1 and S2 utilize the same set of common or switchingfrequencies C₁-C₄. Thus, each central control station utilizes two setsof frequencies, there being four frequencies in each set, resulting in atotal of eight frequencies handled per station.

[0070] Thus, the second embodiment of the invention is suitable for asystem having a plurality of central control stations 420_(x,x=1,2, . . . M). Each central control station 420 _(x) transmits andreceives a set of local frequencies f_(L1), f_(L2), f_(L3), f_(L4) in anassociated geographical area or cell, as well as the set of common orswitch frequencies C₁, C₂, C₃, C₄. While the values of the localfrequencies f_(L1), f_(L2), f_(L3), f_(L4), vary from cell to cell(e.g., differ for differing central control stations 420 _(x)), thevalues of the common or switch frequencies C₁, C₂, C₃, C₄ are uniformthrough the system (e.g., for all central control stations 420 _(x)).

[0071] Although not shown in FIG. 9, it should be understood that thepattern of central control stations repeats in like manner in allcompass directions in accordance with the prescribed geographicalboundaries of the paging system. Moreover, although not specificallyillustrated in FIG. 9, it should also be understood that each centralcontrol station 420 has an associated CFRR.

[0072] The common or switching frequencies C₁-C₄ have an analogousfunction to the corresponding local frequencies f₁-f₄, respectively. Inthis regard, frequency C₁ carries a clock frequency transmitted bycentral control station(s), although the clock rate on common frequencyC₁ preferably varies among central control stations. Frequency C₂ isused to transmit information from central control station(s) to pagerunit(s); frequency C₃ is used to transmit information from a pager unitto a central control station; frequency C₄ is used by pager units toissue a request signal. Frequency C₂ carries packets having a formatsimilar to that of FIG. 12. In analogous manner to frequency f₂, thepackets carried by frequency C₂ may have command codes. Among the C₂command codes are a SYSTEM COMMAND CODE; a LOCAL FREQUENCY DOWNLOADCOMMAND CODE; a SLOT RECOGNITION COMMAND CODE; and a SLOT ASSIGNMENTCOMMAND CODE.

[0073] As shown in FIG. 7, central control station 420 resembles centralcontrol station 20 of the embodiment of FIG. 1 (similar components beingassigned the same reference numerals for simplicity). However, centralcontrol station 420 is augmented by inclusion of a further transmitter,known as common frequency transmitter 432, together with its commonfrequency transmission antenna 442, for transmitting the commonfrequencies C₁ and C₂. In contrast to the high power transmitter 32,transmitter 432 is a low power transmitter. Further, central controlstation 420 is augmented by inclusion of a further receiver, known asthe common frequency receiver 434, together with its common frequencyreceiver antenna 444, for reception of the common frequencies C₃ and C₄.

[0074] Central control station 420 of FIG. 7 includes a clock unit 59′which generates two clocking signals—a first or local clocking signalf_(L)clk and a second or common clocking signal C₁clk. The localclocking signal f_(L)clk is used to modulate frequency f₁); the commonclocking signal is used to modulate the common frequency C₁.

[0075] The central computers 30 of the central control stations ⁴²⁰ _(x)are serially connected to one another by an output line 486A and aninput line 486B. In particular, although not expressly shown as such inFIG. 7, computer 30 of FIG. 7 (like that of FIG. 1) includes an I/Ointerface to which the serial lines 486A and 486B are connected. Seriallines 486A and 486B are used, for example, to update contents of thepager registration file 55 and the pager directory file 56.

[0076] As shown in FIG. 8, pager unit 422 resembles pager unit 22 of theembodiment of FIG. 2 (similar components again being assigned the samereference numerals for simplicity). However, pager unit 422 (in likemanner as central control station 420) is augmented by inclusion of afurther transmitter, known as common frequency transmitter 572, togetherwith its common frequency transmission antenna 576, for transmitting thecommon frequencies C₃ and C₄. Further, central control station 420 isaugmented by inclusion of a further receiver, known as the commonfrequency receiver 434, together with its common frequency receiverantenna 444, for reception of the common frequencies C₁ and C₂.

[0077] The operational frequencies of transmitter 72 and receiver 62 arechangeable in accordance with values transmitted on “frequency control”lines from computer 70. In particular, the frequency control lines areconnected to I/O interface 86 in computer 70. As described in moredetail below, when a pager unit 422 migrates into a new CFRR, signalsare applied on the frequency control lines in order to switch pager unit422 from the local frequencies of an old cell to the local frequenciesof a new cell associated with the new CFRR into which pager unit 422migrates.

[0078] Pager 422 includes a clock unit 83′ which is capable ofseparately generating local clocking signals f_(L)clk and the commonclocking signals f_(c1)clk for use by microprocessor 80. These clockingsignals are initiated and their frequencies set by appropriaterespective inputs to clock unit 83′.

[0079]FIG. 8 also shows that pager unit 422 has data I/O unit 596 whichincludes both an alphanumeric graphic display and a pressure sensitivewriting pad. The alphanumeric graphic display is a dot matrix devicewhich can display characters and graphics. The writing pad has a 16×48dot area.

Operation of Second Embodiment

[0080] As shown in FIG. 9, a pager unit P1 is assumed to have beenoperating in CELL₁ and to have previously received the commonfrequencies C₁-C₄ and local frequencies f₁-f₂ from station S1. Now pagerunit P1 travels on a route indicated by broken arrow-headed line ROUTE.In travelling along the ROUTE, pager unit P1 continues to operate onlocal frequencies f₁-f₂, even as it travels through the cellular overlapregion. However, when page unit P1 enters a new common frequencyreception region (i.e., CFRR₂), a switching or hand-off operationoccurs. In the switching operation, as explained in more detail below,pager unit P1 obtains common frequencies C₁-C₄ from central controlstation S2 and, as a result, can switch from the local frequencies f₁-f₄of CELL₁ to the local frequencies f₅-f₈ of CELL₂. In order to effect theswitching or hand-off operation, pager unit P1 executes a channelswitching routine; the central control station S2 executes a switchingenabling routine.

[0081] In connection with the channel switching routine and theswitching enabling routine, when pager unit P1 moves into CFRR₂. pagerunit P1 will receive the clocking signal on frequency C₁ from stationS2. At such point, pager unit P1 will automatically align its clock unitwith the clocking signal from station S2.

[0082] Referring now to the channel switching routine executed by pagerP1 subsequent to start-up (step 500), at step 506 pager unit P1 obtainsinformation characterizing the system centered about station S2. Suchcharacterizing information is referred to as system identification orsystem ID information.

[0083] At step 508, microprocessor 80 of pager unit P1 checks todetermine if there is any new system ID information acquired onfrequency C₂. That is, microprocessor 80 checks to determine if systemID information is received on frequency C₂ (which can occur only in aCFRR) and, if so, compares the system ID information to the immediatelypreviously-stored system ID information. If the previous and mostrecently-acquired system IDs are the same, pager unit P1 realizes thatit is still in the jurisdiction of the same station (e.g., station S1).If not, pager unit P1 realizes that it has now wandered into a CFRR of anew station (e.g., station S2) and, at step 510, initiates a request onfrequency C₄ for communication with the central control station (e.g.,station S2) for CELL₂.

[0084] In the above regard, since pager unit P1 has not yet beenassigned a time slot for CELL₂, the request on frequency C₄ is randomlymade. However, pager unit P1 keeps track of the time slot in which itmakes its request to the new central control station (e.g., station S2).

[0085] Thereafter, pager unit P1 continues to monitor (step 512)communications packets from station S2 on frequency C₂, waiting forstation S2 to issue a message which references the time slot at whichpager unit P1 made its request of step 510. In particular, page unit P1awaits a message from station S2 on frequency C₂ that includes both aSLOT RECOGNITION COMMAND CODE and information stored in the same timeslot which pager unit P1 randomly generated. Since the message includingthe SLOT RECOGNITION COMMAND CODE includes station S2 as the sender andmirrors the slot randomly generated by pager unit P1, pager unit P1recognizes the message as being addressed to pager unit P1 and considersissuance of such a message by station S2 (see step 612 of FIG. 11) toconstitute authority for pager unit P1 to communicate further withstation S2. In this regard, at step 514 microprocessor 80 of pager unitP1 determines if there is a match between the time slot of a receivedmessage and the time slot at which the random request was made at step510.

[0086] Assuming a match is eventually found at step 514, at step 516pager unit P1 sends a communications packet on frequency C₃ to stationS2, with the communications packet including the identification or ID ofpager unit P1. Using pager registration file 55, station S2 verifiesthat the ID of pager unit P1 is a valid ID, and thereafter sends (onfrequency C₂) to pager unit P1 a message with the command code LOCALFREQUENCY DOWNLOAD, which message informs pager unit P1 of the values ofthe local frequencies handled by station S2 (e.g., frequencies f₅-f₈).Thereafter, as also reflected by step 518, station S2 sends (onfrequency C₂) to pager unit P1 a message with the command code SLOTASSIGNMENT COMMAND CODE, which message informs pager unit P1 of its slotassignment on frequency f₈. Microprocessor 80 then changes its slotallocation by steps which are similar to those discussed with theafore-mentioned change time slot routine (see steps 350, 352, and 354 ofFIG. 5). Step 518 of FIG. 10 reflects reception of the local frequencyvalues and reception of the slot assignment.

[0087] After acquisition of all local frequencies and the slotassignment is completed (step 520), microprocessor 80 implements (atstep 522) a switch to the new local frequencies (e.g., frequenciesf₅-f₈). In this regard, microprocessor 80 instructs I/O interface 86 tochange transmitter 72 from frequencies f₃, f₄ to frequencies f₇, f₈; andto change receiver 62 from frequencies f₁, f₂ to frequencies f₅, f₆. I/Ointerface 86 accomplishes the frequency changes by applying appropriatevalues on the frequency control lines connecting the I/O interface totransmitter 72 and receiver 62, respectively.

[0088] After the switch to new local frequencies at step 522,microprocessor 80 loops back to step 506, ultimately to determine whenany further switching may be required.

[0089] Steps involved in the switching enabling routine executed by acentral control station (e.g., station S2) are depicted in FIG. 11.After start-up (step 600), CPU 50 determines executes a loop 602 whichenables CPU 50 to clean up its pager directory file 56 and to check ifany new pager units have wandered into the cell which it administers.

[0090] In particular, at step 604 CPU determines whether its centralcontrol station (e.g., S2) has been advised by any other central controlstation (e.g., S3) that a pager unit, formerly under the control of itscentral control station (e.g., S2), has come under the control of theother central control station (e.g, S3). Such advisement occurs on theserial links connecting the central control stations 420 _(x), andparticularly input serial link 486B. If such advisement occurs, the IDfor the wandered-away pager is deleted from the pager directory file 56for station S2 (as reflected by steps 606 and 608).

[0091] At step 610, CPU 50 causes messages with a SYSTEM COMMAND CODE tobe transmitted on frequency C₂. As indicated before, messagestransmitted on frequency C₂ include a packet(s) having a format such asthat shown in FIG. 12. The message with the SYSTEM COMMAND CODEparticularly includes the central station ID number in its alphanumericdata field.

[0092] At step 612, central control station 420 checks to determine if arequest signal has been transmitted by any pager unit 422 on frequencyC₄ (as occurred, for example, in context of the discussion of FIG. 10,particularly step 510). Such a request signal would likely be issuedfrom a pager unit 422 which has just wandered into the CFRR controlledby the central control station (e.g., into CFRR₂ controlled by stationS2). If no such request signal is detected, loop 602 is again repeated.

[0093] In the event that a request signal is detected at step 612,central control station 420 notes specifically the time slot onfrequency C₄ at which the request occurred (step 614). At this point,such time slot is the only way central control station 420 can identifythe in-wandering pager unit 422. Central control station 420 desires forthe in-wandering pager unit 422 to transmit its identification (ID), butcannot specifically address the in-wandering pager other than withreference to the detected time slot. Accordingly, at step 616, centralcontrol station 420 prepares and transmits a message on frequency C₂which has a SLOT RECOGNITION COMMAND CODE. The message including theSLOT RECOGNITION COMMAND CODE includes station S2 as the sender andmirrors the slot randomly generated by pager unit P1 (e.g, the time slotat which the in-wandering pager unit 422 issued its request). Thistransmission on frequency C₂ constitutes authority for pager unit P1 totransmit its identification.

[0094] Step 618 denotes acquisition by central control station 420 ofthe identification (ID) of the in-wandering pager unit 422. At step 620,central control station 420 checks its pager registration file 55 todetermine if the pager ID is a valid ID. If not, an error message isgenerated and transmitted (at step 622), followed by a command for pagerunit P1 to shut down (see step 624).

[0095] Assuming that the identification of pager unit 422 was validatedat step 620, CPU 50 checks (at step 630) its pager directory file 56 tolocate an available time slot for the in wandering pager unit 422, andthen associates the available time slot with the ID of the in-wanderingpager unit 422. Then, at step 632, using a message on frequency C₂ witha LOCAL FREQUENCY DOWNLOAD COMMAND CODE, central control station 420sends the values of its local frequencies (e.g., f₅, f₆, f₇, f₈) to thein-wandering pager unit 422. The central control station then (at step634) assigns to the in-wandering pager unit 422 a new time slot on itslocal frequencies using a message on frequency C₂ with a SLOT ASSIGNMENTCOMMAND CODE. Processing of the change time slot command by thein-wandering pager unit 422 is understood with analogous reference toFIG. 5, particularly steps 350, 352, and 354.

[0096] Upon completion of step 634, the in-wandering pager unit 422 isfully initiated into its new cell (e.g., CELL₂), and has left thejurisdiction of its former control station (e.g, CELL₁ and station S1).Accordingly, at step 636, CPU 50 requests its I/O interface to issue acommand on serial line 486A which advises (using pager ID) that thein-wandering pager 422 is now under its jurisdiction, so that formerjurisdictions (e.g., S1) can delete this pager unit from their pagerdirectory files 56. Such deletion is understood with reference to steps604-608 as above-described.

[0097] In addition to illustrating geographical location of pager P1,stations S1 and S2, and cells CELL₁ and CELL₂, FIG. 9 shows the relativetiming of communications occurring on common frequencies C₁-C₄. FIG. 9specifically relates the timing of communications transmissions tospecific ones of the aforedescribed steps executed by central controlstation 420 (the switching enabling routine of FIG. 11) and by pagerunit 422 (the channel switching routine of FIG. 10).

[0098] Although the central control stations 420 _(x) use the samecommon frequencies C₁-C₄, there is no interference or confusion of thesesignals transmitted from the control stations 420 _(x). The commonfrequencies C₁-C₄ are broadcast at a relatively lower power than thelocal frequencies f₁-f₄ so that reception of the common frequenciesC₁-C₄ occurs only in a limited neighborhood (CFRR) about the centralcontrol station 420 _(x). Accordingly, pager units 422 traveling throughthe system receive common frequencies C₁-C₄ only in the limited andnon-overlapping CFRRs.

[0099] System operational characteristics, such as cell diameter, CFRRdiameter, power level of the local frequencies (e.g., f₁-f₄), and powerlevel of the common frequencies (C₁-C₄) can be field adjusted to suitnumerous factors, including particularly the terrain and topography ofthe geographical region covered by the system. By way of non-limitingexample, in one embodiment, the radius of each cell is on the order ofabout 20 miles; while the radius of each CFRR is on the order of about10 miles or less. In the same example, the power for transmission of thelocal frequencies can be in a range of from about 3 watts to 1000 watts;while the power for transmission of the common frequencies C₁-C₄ ispreferably less than 2 watts.

[0100] Thus, the invention provides a two-way paging system whichoperates independently from a telephone system for wireless datacommunication between users. The invention minimizes use of availablefrequencies allowed by the Federal Communications Commission (FCC),using only four local frequencies f₁-f₄ for any given cell and (forexpanded, multi-cellular coverage) only four common or switchingfrequencies C₁-C₄. In order to minimize the number of frequencies (e.g,channels) utilized, techniques of time division sharing andsynchronization are employed. A transmission power differential betweenthe local frequencies and the common frequencies is also employed. Thesetechniques allow data transmission to be kept separate from differentpagers and thus eliminates merging of data.

[0101] The switching technique of the present invention providesextended geographical coverage and minimizes paging time by increasingthe number of frequencies utilized in a cell from four (e.g, the fourlocal frequencies) to eight (the four local frequencies plus the fourcommon frequencies).

[0102] In connection with verification of pager ID, it should beunderstood that a single pager registration file might be stored in amemory file only one of a plurality of central control stations, andthat in such case verification would constitute issuing a search command(on the serial links 486) to locate a pager ID in the one (remote)memory file, with the results of the search being reported back to theinquiring central control station.

[0103] The keyboards illustrated herein can, in some embodiments, bemulti-language keyboards or writing pads which permit typing of English,Chinese, or Japanese languages, for example. The writing pad isespecially useful in countries such as Japan, Thailand, the middle Eastor China where English-like alphabets are not used. The writing padcould also be used to sketch and transmit graphics. Moreover, datacompression/de-compression techniques can be utilized in connection withdata transfer.

[0104] While the invention has been particularly shown and describedwith reference to the preferred embodiments thereof, it will beunderstood by those skilled in the art that various alterations in formand detail may be made therein without departing from the spirit andscope of the invention. For example, it should be understood thatrepeaters may be employed within cells to facilitate transmission when apager unit ventures far from a central control station.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method of operating apaging system, the paging system including a central control station anda paging unit, the method comprising: transmitting, on a firstfrequency, a clock-aligning signal from the central control station tothe paging unit; transmitting, on a second frequency, a pager commandand alphanumeric data from the central control station to the pagingunit; transmitting, on a third frequency, and in response to the pagercommand, pager status data and alphanumeric data from the paging unit tothe central control station; transmitting, on a fourth frequency, apager transmission request signal from the paging unit to the centralcontrol station, the pager transmission request signal being transmittedin a predetermined time slot assigned to the paging unit, thepredetermined time slot being related to the clock-aligning signal andassigned whereby the fourth frequency is utilizable by a plurality ofother paging units; wherein the first frequency, second frequency, thirdfrequency, and fourth frequency differ from one another.
 2. A method ofoperating a paging system wherein a paging unit acquires radiocommunication with a control station, the method comprising:transmitting a clock-aligning signal from the control station; using theclock-aligning signal to align therewith a clock of the paging unit;transmitting from the control station a station identification messagewhich includes station identifying information; determining if thestation identifying information received at the paging unit has changedand, upon determination of such change executing the following steps:generating, at the paging unit, a station switch request signal, thestep of generating the station switch request signal includinggenerating a frame of information including a plurality of time dividedslots in accordance with the clock-aligning signal, and selecting one ofthe plurality of time slots as a time slot to be at least temporarilyassociated with the paging unit; receiving the station switch requestsignal at the control station and responsively transmitting from thecontrol station an authorization message, which authorization messageauthorizes the paging unit to engage in further communications;transmitting, in response to reception of the authorization message, apaging unit identification message which includes pager identificationinformation of the paging unit; downloading, to the paging unit from thecontrol station, a set of local frequencies for further communicationbetween the paging unit and the control station.
 3. The method of claim2, wherein the control station transmits and/or receives messages on afirst common frequency, a second common frequency, a third commonfrequency, and a fourth common frequency, and wherein the clock-aligningsignal from the second control station is transmitted on the firstcommon frequency, wherein the station identification message istransmitted on the second common frequency; wherein the station switchrequest signal is transmitted on the fourth common frequency; andwherein the authorization message is transmitted on the third commonfrequency.
 4. The method of claim 2, wherein the transmissionauthorization message includes a frame of information including a sameplurality of time divided slots in accordance with the clock-aligningsignal as in the station switch request signal, and wherein informationis stored in a same time slot as in the station switch request signal.5. The method of claim 2, wherein, in response to reception of thepaging unit identification message, the control station determineswhether the pager identification information included in the paging unitidentification message is valid.
 6. The method of claim 2, wherein afirst local frequency is used to transmit a local clock-aligning signalfrom the central control station to the paging unit; wherein a secondlocal frequency is used to transmit a pager command and alphanumericdata from the central control station to the paging unit; wherein athird local frequency is used to transmit alphanumeric data from thepaging unit to the central control station; and wherein a fourth localfrequency is used to transmit a transmission request signal from thepaging unit to the central control station.
 7. The method of claim 6,wherein the transmission request signal is transmitted in apredetermined time slot assigned to the paging unit, the predeterminedtime slot being related to the local clock-aligning signal and assignedwhereby the fourth local frequency is utilizable by a plurality of otherpaging units.
 8. The method of claim 2, wherein the first localfrequency, second local frequency, third local frequency, and fourthlocal frequency differ from one another.
 9. A two-way paging unitcapable acquiring radio communication with a control station, the pagingunit comprising: a first receiver for receiving a clock-modulated firstfrequency transmitted by the control station; a clocking unit and aclock alignment circuit which aligns the clocking unit with theclock-modulated first frequency; a second receiver which receives asecond frequency, the second frequency being at least intermittentlymodulated to include station identifying information which identifiesthe control station; a processor which determines if station identifyinginformation received by the processor has changed and which, upondetermination of such change, generates a station switch request signal,the station switch request signal including a frame of informationcomprising a plurality of time divided slots related to theclock-modulated first frequency, and wherein one of the plurality oftime slots is selected by the processor as a time slot to be at leasttemporarily associated with the paging unit; a transmitter fortransmitting the station switch request signal to the central station.10. The apparatus of claim 9, wherein the processor further detectsreceipt of a transmission authorization message and in response theretocauses a transmission of a paging unit identification message.
 11. Theapparatus of claim 10, wherein the transmission authorization message isdetected as having a frame of information including a same plurality oftime divided slots as in the station switch request signal, and whereininformation is stored in a same time slot as in the station switchrequest signal.
 12. The apparatus of claim 10, further comprising atransmitter for transmitting the paging unit identification message. 13.The apparatus of claim 9, wherein the paging unit receives from thecontrol station a downloaded set of local frequencies for use in furthercommunication between the paging unit and the control station.
 14. Acontrol station which communicates with a two-way paging unit, thecontrol station comprising: a clock unit which generates at least afirst clocking signal; a first receiver which receives a station switchrequest signal from the paging unit, the station switch request signalincluding a frame of information comprising a plurality of time dividedslots related to the first clocking signal, one of the plurality of timeslots as selected by the paging unit bearing information serving to atleast temporarily associate with the paging unit with the selected timeslot; a processor which prepares a paging unit transmit authorizationsignal, the paging unit transmit authorization signal including a frameof information comprising a same plurality of time divided slots as thestation switch request signal and information in a same selected one ofthe time slots as in the station switch request signal; a transmitterfor transmitting the paging unit transmit authorization signal.
 15. Theapparatus of claim 14, wherein the processor of the control stationgenerates a local frequency download message for downloading a set oflocal frequencies for use in further communication between the pagingunit and the control station, and wherein the transmitter transmits thelocal frequency download message.
 16. The apparatus of claim 15, whereinthe clock unit generates a second clocking signal for use as a localclocking signal, and wherein the local clocking signal is transmitted ona first of the local frequencies.
 17. The apparatus of claim 14, whereina first local frequency is used to transmit a local clock-aligningsignal from the central control station to the paging unit; wherein asecond local frequency is used to transmit a pager command andalphanumeric data from the central control station to the paging unit;wherein a third local frequency is used to transmit alphanumeric datafrom the paging unit to the central control station; and wherein afourth local frequency is used to transmit a transmission request signalfrom the paging unit to the central control station; wherein thetransmission request signal is transmitted in a predetermined time slotassigned to the paging unit, the predetermined time slot being relatedto the local clock-aligning signal and assigned whereby the fourth localfrequency is utilizable by a plurality of other paging units.
 18. Theapparatus of claim 17, wherein the processor of the control stationgenerates a slot assignment message for downloading a set of localfrequencies for use in further communication between the paging unit andthe control station, and wherein the transmitter transmits the localfrequency download message.
 19. The apparatus of claim 14, wherein inresponse to the paging unit transmit authorization signal the controlsstation receives a paging unit identification message which includespaging unit identification information, and wherein the processordetermines whether the paging unit identification information is valid.20. A control station which communicates with a paging unit, the controlstation comprising: a first transmitter for transmitting a set of localfrequencies to a cell region associated with the control station; asecond transmitter for transmitting a set of switching signalfrequencies to a switching region associated with the control station; aclock unit for generating a local clock signal and a switching clocksignal; a processor for generating message information and switchinginformation; and wherein a first of the local frequencies is modulatedto carry the local clock signal, a second of the local frequencies ismodulated to carry the message information; a first of the switchingsignal frequencies is modulated to carry the switching clock signal; anda second of the switching signal frequencies is modulated to carry theswitching information.
 21. The apparatus of claim 20, wherein the firsttransmitter is operated at a greater power than the second transmitter.22. The apparatus of claim 20, wherein the cell region has a greatergeographical extent than the switching region.
 23. The apparatus ofclaim 20, further comprising: a first receiver which receives a stationswitch request signal from the paging unit, the station switch requestsignal including a frame of information comprising a plurality of timedivided slots related to switching clock signal, one of the plurality oftime slots as selected by the paging unit bearing information serving toat least temporarily associate with the paging unit with the selectedtime slot; wherein the processor prepares a paging unit transmitauthorization signal, the paging unit transmit-authorization signalincluding a frame of information comprising a same plurality of timedivided slots as the station switch request signal and information in asame selected one of the time slots as in the station switch requestsignal; and wherein the first transmitter transmits the paging unittransmit authorization signal.
 24. The apparatus of claim 23, whereinthe processor of the control station generates a local frequencydownload message for downloading the set of local frequencies for use infurther communication between the paging unit and the control station,and wherein the first transmitter transmits the local frequency downloadmessage.
 25. The apparatus of claim 23, wherein the second of the localfrequencies is used to transmit a pager command and alphanumeric datafrom the central control station to the paging unit; wherein a third ofthe local frequencies is used to transmit alphanumeric data from thepaging unit to the central control station; and wherein a fourth of thelocal frequencies is used to transmit a transmission request signal fromthe paging unit to the central control station; wherein the transmissionrequest signal is transmitted in a predetermined time slot assigned tothe paging unit, the predetermined time slot being related to the localclock signal and assigned whereby the fourth of the local frequencies isutilizable by a plurality of other paging units.
 26. The apparatus ofclaim 25, wherein in response to the paging unit transmit authorizationsignal the controls station receives a paging unit identificationmessage which includes paging unit identification information, andwherein the processor determines whether the paging unit identificationinformation is valid.